A Level Biology Revision Notes

Comprehensive A Level Biology revision notes providing information and assistance for all UK examination boards (AQA, OCR, Edexcel) as well as international curriculum (CIE).

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Temporal and Spatial Summation

Introduction Neurons are continuously receiving from thousands of other neurons around it. However, whether these inputs are able to elicit an action potential or not depends on the summation of these inputs. Summation can be defined as a process by which the excitatory and inhibitory signals together are able to generate an action potential or …

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Repolarization

Introduction Repolarization is the phase that follows depolarization. During an action potential, the first stage is depolarization in which sodium ion channels open causing an influx of sodium ions into the neuron. This causes the membrane potential to reach approximately +40mV from a resting membrane potential of -70mV. At this membrane potential of about +40mV, …

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Neuromuscular Junction

Introduction The neuromuscular junctions play the important role of transferring action potentials to the skeletal muscle fibers. Fibers of the skeletal muscles are innervated by large, myelinated nerve fibers that originate in the large motor neurons in the anterior horn of the spinal cord. All of these fibers branch out to innervate anywhere from between …

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Nerve Impulse

Introduction Nerve impulse was discovered by British Scientist Lord Adrian in the 1930s. Owning to the importance of this discovery, he was awarded Noble Prize in 1932. Nerve Impulse is a major mode of signal transmission for the Nervous system. Neurons sense the changes in the environment and as a result, generate nerve impulses to …

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Myelinated and Unmyelinated Axons

Introduction Nerve fibers or axons are a part of the nervous system which receives environmental signals and produce a response. For response, the nervous system uses nerve fibers for transmitting nerve impulses to the target cells. These nerve fibers are of three types: Sensory neurons Interneurons Motor neurons These neurons are classified into two categories …

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Differential Permeability

Introduction Differential permeability is defined as a property of cellular membranes that allows only selective substances to enter or leave the cell. This feature of cellular membranes helps to maintain a constant internal environment regardless of the changes in the external environment. Many ions, water, glucose, and carbon dioxide are constantly imported and exported from …

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Myelinated Motor Neurons

Introduction Neurons are specific cells of the Nervous system that are involved in transmitting signals from the brain and spinal cord to target cells of the body. It is estimated that the number of neurons in the human brain is about 80 billion. These neurons are classified into the following three types: Sensory neurons Interneurons …

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Electrochemical Gradients

Introduction In living cells, the plasma membrane or cell membrane is a selectively permeable barrier that allows selective substances to pass through it. Thus, it maintains different concentrations on both sides of the membrane. This gives rise to different electrical and chemical concentration gradients on the membrane surface which collectively form the electrochemical gradient. What …

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Action Potential

Introduction Certain cells in the body are electrically active and can relay and sustain voltage fluctuations. These voltage fluctuations allow the propagation of signals and can be thought of as a means of communication between the cells. How these fluctuations come about is to a large extent dependent upon the cell membrane and the movement …

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Hyperpolarization

Introduction Hyperpolarization is said to be the final stage of an action potential after depolarization and repolarization in an action potential, respectively. Many diseases and conditions may arise from dysfunctions and mutations in the hyperpolarization activated cyclic nucleotide gated (HCN) channels. These disorders include Parkinson’s disease, Alzheimer’s disease, amyotrophic lateral sclerosis among the neurodegenerative disorders. …

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